This invention relates to power control systems for wireless terminals.
For the reverse link of the CDMA cellular system defined by IS-95 standard, a closed-loop power control scheme is used to maintain the uniform received signal power at the base station receiving antenna. Based on the measurement of the received signal energy at the base station, the base station sends back power control bit streams at 800 bits/second rate. The mobile station adjusts the transmission power up or down a fixed power step according to the power control bit. Due to the impairments in the power control bits transmission and generation (errors and delay), this power control scheme still results in non-negligible variation in receiving power at the base station in the presence of channel fading and shadowing, thus affecting the overall system performance.
Since the closed-loop power control is mainly for compensation of the small scale fading caused by the moving of the vehicle in which the mobile station is installed, and the Doppler spread of the fading is relatively small compared to the power control bit rate, the inventors proposes using the correlation of channel gains between successive power control groups to relieve the impact of the impairments in the power control bit transmission and generation.
In this proposal, a (linear) predictor is used to predict the accumulation of the received power control bits received by the mobile station, and use the difference of two successive predictor outputs to adjust the output power of the mobile in a multi-level (or continuous) increments fashion.
In one aspect of the invention, there is provided a wireless terminal for use in a communications system using a power control bit channel, comprising:
a receiver tunable to the power control bit channel, the receiver having, in operation, a received signal as output;
an accumulator for accumulating the output of the receiver to produce an accumulated signal;
a predictor having as input the accumulated signal and output a prediction of future values of the accumulated signal;
a power controller having as input the output from the predictor and having as output a signal whose power is a function of the predictor output; and
a transmitter for transmitting signals having a power that is a function of the predictor output.
In a further aspect of the invention, there is provided a method of controlling power of transmitted signals exchanged between first and second wireless terminals, the method comprising the steps of:
an accumulating in an accumulator a received signal to produce an accumulated signal;
predicting in a predictor future values of the accumulated signal; and
controlling the power of a signal transmitted by the wireless terminal based on a function of the future values output from the predictor.
Preferably, a differential decoder is located between the predictor and the power controller, so that the function of the future values is a function of the difference between successive future values of the output from the predictor. Preferably, the predictor is reset when the difference between successive outputs from the predictor exceeds a given value. The function of the future values may be a discontinuous or continuous function. A preferred implementation of the predictor includes time variable tap weight coefficients and the predictor implements a Widrow least mean square algorithm to recursively adjust the tap weight coefficients.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.